SPOKANE, Wash. (AP) - Workers are trying to determine how to clean up one of the worst radioactive waste leaks in years at the Hanford nuclear reservation, officials said.

No workers were contaminated during last week's accident and the spill was contained within a tiny area, posing no threat to the public, officials said Tuesday.

The leak was estimated at between 50 and 100 gallons, although officials are not yet sure how big it was, Delmar Noyes, of the U.S. Department of Energy, told reporters during a conference call.

The spill area has been capped to prevent the waste from becoming airborne. A plan to safely dispose of the spill is being developed.

"The release to the environment of this waste material is not acceptable," Noyes said.

The spill, which Noyes said was the largest in the tank farm in years, illustrates the difficulties of trying to safely dispose of nuclear waste that dates back to World War II.

The spill was believed to have occurred early Friday, but was not detected until about 10 a.m., some seven hours later, Hanford officials said.

A watchdog group criticized the Energy Department for what it called a slow response to the leak.

"The failure to detect the leak for hours overnight, while deadly high-level nuclear wastes apparently spilled onto the ground, raises serious questions requiring state and federal investigations," said Gerald Pollet, executive director of Heart of America Northwest.

Hanford officials contend they notified regulators in an appropriate fashion after the release was discovered.

The spill occurred as an underground tank was being slowly drained of its nuclear waste, which since 2004 has been pumped into newer, double-walled tanks that are less likely to leak.

The waste from the bottom of the tank is so lethal "that a cup full of waste would kill everyone in a room in a short period of time," Pollet said.

Early Friday, the pump became clogged and workers reversed it in an effort to clear the blockage. That sent some waste from the bottom of the tank up into the hose that was feeding water into the tank, leading to the leak onto the ground, Noyes said.

Workers in surrounding areas were evacuated and the pumping operation was shut down. Also shut down was the pumping of another nuclear waste storage tank. Both will remain closed until it is determined that work can safely proceed.

Hanford covers about 560 square miles in south-central Washington, and contains the nation's largest collection of nuclear waste from the production of weapons.